The present invention generally relates to high-speed data communications on a transmission line. More specifically, the invention relates to an improved method and apparatus for active line termination.
With the advancement of technology, and the need for instantaneous information, the ability to transfer digital information from one location to another, such as from a central office (CO) to a customer premise (CP) has become more and more important.
In a digital subscriber line (DSL) communication system, and more particularly an xDSL system where xe2x80x9cxxe2x80x9d indicates a plurality of various standards used in the data transfer (e.g., ADSL, VDSL, SDSL, etc.), data is transmitted from a CO to a CP via a transmission line, such as a two-wire pair, and is transmitted from the CP to the CO as well, either simultaneously or in different communication sessions. The same transmission line might be utilized for data transfer by both sites or the transmission to and from the CO might occur on two separate lines.
An xDSL communication system utilizes an amplifier, commonly termed a line driver, to amplify the transmit signal in order to drive it across a transmission line, where the transmission line spans a certain distance. The transmit signal must be amplified to increase the power of the signal in order to overcome the losses caused by the characteristic impedance of the line. As transmission speeds have increased, the need for highly-linear components to reduce distortion of the signal has grown. One way to reduce distortion of the signal is to reduce the reflections of the transmit signal caused by impedance mismatching, particularly between the output impedance of the line driver and the characteristic impedance of the line. FIG. 1 illustrates a line driver circuit 1 in which the output impedance of the line driver is matched to the characteristic impedance (or commonly called load impedance). This has been conventionally accomplished as diagrammatically illustrated in FIG. 1 by terminating the output 3 of a driver amplifier 2 with a line-coupling output resistor 4, the value Rs of which is set equal to the impedance (e.g., 135 ohms, as a non-limiting example) of a load 5. The resulting voltage divider formed by output resistor 4 and line impedance 5 dissipates and therefore wastes half the driver""s output power in the output impedance 4. This implies that for each volt of signal swing to be imparted to the line (load impedance 5) a two-volt swing is required at the output 3 of the amplifier 2.
To avoid using a large output resistor, a method known as active termination can be utilized. Active termination emulates a back-matching impedance, which, in series with a small output resistor, equals the output impedance of the line driver seen from the perspective of the transmission line looking into the output of the line driver. This reduces the need for a large output resistor, which, in turn, reduces the need for more power without reducing the effective power amplified on the transmit signal. So, with active termination, a transmit signal can be driven across a line, with reduced power supplied to the driver.
Generally, there are two main methods of active termination known in the art: xe2x80x9coutput voltage sense positive feedbackxe2x80x9d and xe2x80x9coutput current sense negative feedback.xe2x80x9d Positive feedback is the more popular method, but has several drawbacks. As the various resistance values of the positive feedback network are changed, the behavior of the circuit will qualitatively change as the signs (not only the magnitude) of the equivalent line driver gain, A0, and the apparent back-matching resistance, ROUT, change. These breaks or critical points in the functions defining both variables are characteristic of positive feedback systems. Positive feedback, in addition to introducing the qualitative changes noted above, also tends to emphasize or exaggerate component imperfections, system noise, and signal distortion.
Negative feedback active termination may avoid the drawbacks of the positive feedback configuration, but typically, requires an additional xe2x80x9csense amplifierxe2x80x9d which leads to a more cumbersome and complex design, as well as increasing the overall cost of the line driver.
Accordingly, there is a need for a method and apparatus for active termination line driver with improved power efficiency that overcomes the drawbacks of the prior art.
The present invention relates to an improved method and apparatus for active line termination. In this regard, an active termination line driver (ATLD) configured to drive a transmit signal across a load is provided. The ATLD includes a pair of power amplifiers configured to amplify a transmit signal, the amplifiers comprise a first input for receiving the transmit signal, a second input for receiving a feedback signal, and an output configured to provide the amplified transmit signal to the load. The ATLD also includes a resistive network configured to provide the feedback signal from the outputs of the amplifiers to the second inputs of the power amplifiers. The resistive network is selectively configured to facilitate any one of a plurality of feedback configurations to emulate a back-matching impedance.
In another embodiment, an ATLD includes a differential amplifier configured to amplify a transmit signal, the differential amplifier comprises a pair of inputs for receiving the transmit signal and a pair of outputs configured to provide the amplified transmit signal to the load. The ATLD also includes a resistive network configured to provide a feedback signal from the outputs of the differential amplifier to the inputs of the differential amplifier. The resistive network is selectively configured to facilitate any one of a plurality of feedback configurations to emulate a back-matching impedance.
Embodiments of the invention may also be provided as methods for power-efficiently driving a transmit signal to a load. One method comprises the steps of: selectively configuring a resistive feedback network comprised in a line driver, wherein the resistive feedback network is configured to facilitate any one of a plurality of feedback configurations; applying a transmit signal to an input of the line driver; amplifying the transmit signal; applying the amplified transmit signal to the load; sensing at least a first electrical property generated by amplifying the transmit signal; and applying at least the first electrical property to the passive resistive feedback network to generate a feedback signal responsive to at least the first electrical property such that an output impedance emulates a back-matching resistor.